1. Field of the Invention
The present invention relates to an image processing method and apparatus for reproducing a visible image from an image signal obtained by photoelectrically reading an image recorded in a reflective original such as a photographic print or any of other printed matters, or a transparent original such as a film, or from an image signal obtained by photographing an original image with a digital camera or the like using an image pickup device such as a charge-coupled device (CCD).
2. Description of the Related Art
Presently, the dominating method of printing on a photosensitive material (photographic paper) an image photographed on a photographic film such as a negative film or a reversal film (hereinafter referred to simply as a film) is a direct exposure (analog exposure) method in which an image formed on a film is projected onto a photosensitive material to perform surface exposure on the photosensitive material.
On the other hand, digital photoprinters have recently been put to practical use as a printing apparatus using digital exposure. That is, in the digital photoprinters, an image formed on a film is photoelectrically read and converted into a digital signal, which is processed by various kinds of image processing to generate a recording image signal, and an image (latent image) is formed on a photosensitive material by exposing the photosensitive material to a scanning beam of recording light which is modulated with the image signal, thereby obtaining a printed image (finished image).
The digital photoprinter is capable of determining exposure conditions at the time of printing by processing a digital image signal generated from an image. Therefore, it can suitably perform correction of an excessive light or dark condition of an image resulting from a flash photography condition or a strongly backlighted condition, sharpness (sharpening) processing, correction of a color failure and a density failure, correction of a result of underexposure or overexposure, correction of a reduction in peripheral light quantity, etc., to obtain a print at high quality level not attainable by the conventional direct exposure. The digital photoprinter can also perform synthesis of a plurality of images, division of an image, and synthesis of letters by image signal processing, and can output prints controllably edited and processed according to uses.
Moreover, the digital photoprinter can not only output an image in the form of a print (photograph) but also supply an image signal to a computer or the like or store the image signal on a recording medium such as a floppy disk to use the image signal in various uses as well as in photographic uses.
Basically, the above-described digital photoprinter is constituted by a scanner (image reader) which photoelectrically reads an image recorded on a film, an image processor which processes the read image to generate a recording image signal (exposure conditions), and a printer (image recorder) which forms a print by performing scanning exposure of a photosensitive material according to the image signal and by performing development on the photosensitive material.
In the scanner, light emitted from a light source is made incident on a film to be formed into, projected light carrying the image recorded on the film, and the projected light is imaged on an image sensor such as a CCD sensor and is photoelectrically converted into an electrical signal, thereby reading the image. The electrical signal undergoes various kinds of image processing according to one ""s need to be obtained as a film image signal, which is supplied to the image processor.
The image processor sets image processing conditions from the image signal read by the scanner, processes the image signal under the set conditions to generate an output image signal (exposure conditions) for image recording, and sends the signal to the printer.
If the printer is, for example, a unit for performing light beam scanning exposure, it performs two-dimensional scanning exposure (printing) of a photosensitive material with a light beam by modulating the light beam according to the image signal sent from the image processor, thereby forming a latent image. The printer then performs a predetermined development process, etc., to output a print (photograph) in which the image recorded on the film is reproduced.
Ordinarily, the photographic conditions of images photographed on films vary and there are many cases where the difference between the maximum and minimum levels of light (density) is considerably large in photography using flashlight or in the presence of backlight, in other words, the dynamic range of an image is extremely wide.
If an image photographed on a film under such a condition is printed by performing an ordinary exposure process, there is a possibility of a light (highlighted) or dark (shadow) image portion becoming so light or dark that details thereof are lost. For example, in a case where a human figure in a backlighted state is photographed, a light portion, e.g., an image of a sky is so light that details thereof are lost if exposure is controlled so as to optimize the image of the human figure, or the image of the human figure is so dark that details thereof are lost if exposure is controlled so as to optimize the image of the sky.
For this reason, so-called dodging is performed when exposure of a photosensitive material is performed for printing of an original image which is a film image having a large variation from a highlighted portion to a shadow portion (a wide dynamic range).
Dodging is a technique for correcting the large variation from highlight to shadow in an image photographed on a film to obtain a print in which the image is suitably reproduced throughout the entire image area so that the image is closer to a viewer""s impression of the original image. Correction of the variation from highlight to shadow is performed such that ordinary exposure is performed on a portion having an intermediate density, the amount of exposure of a light portion from which an excessively light image with loss of detail can result easily is increased, and the amount of exposure of a dark portion from which an excessively dark image with loss of detail can result easily is reduced. That is, dodging is a technique for compressing the dynamic range of an original image.
Various ideas of processing corresponding to the conventional dodging have been proposed with respect to the above-described digital photoprinter that reproduces an image on the basis of a digital image signal. Examples of such ideas are image processing methods and apparatuses disclosed in Japanese Patent Application Laid-open Nos. 09-18704, 09-182093, and 10-13679.
Japanese Patent Application Laid-open No. 09-18704 discloses a method of reducing the dynamic range of an original image such that an image signal representing the original image and a signal representing an unsharp image obtained from the original image (unsharp image signal) are processed to obtain a signal representing the difference therebetween, and this difference signal is processed by predetermined image processing to obtain a processed image signal, which is reproduced as a visible image.
Japanese Patent Application Laid-open No. 09-182093 discloses a method in which the unsharp image described in Japanese Patent Application Laid-open No. 09-18704 is formed by filtering through an infinite impulse response (IIR) filter. Japanese Patent Application Laid-open No. 10-13679 discloses a method in which a plurality of low-pass filters are provided, one of the plurality of low-pass filters having the mask size determined according to the pixel density of the reproduced image is selected to form an unsharp image of a varying mask size, and an unsharp image is formed by using the selected low-pass filter.
Each of these processing methods comprises a process in which a setting of the ratio of compression of the dynamic range of the whole of an image, a setting of compression of the dynamic range according to a degree of importance of a light or dark image portion, etc., are made in a setup section, and according to these settings, signal processing is performed as shown in FIG. 9A. That is, a low-frequency component of an image signal S0xe2x80x2, i.e., an unsharp image signal S1xe2x80x2 which changes moderately, is prepared by filtering using a low-pass filter, and the value of the unsharp image signal S1xe2x80x2 is inverted by conversion of the unsharp image signal S1xe2x80x2 based on a look-up table (LUT computation), thereby generating a processed image signal S4xe2x80x2 having a compressed dynamic range. This processed image signal S4xe2x80x2 is added to the image signal S0xe2x80x2 of the original image to obtain an image signal S5xe2x80x2. Thus, the dynamic range of an image signal of a low-frequency component having a frequency lower than the set low-pass filter cutoff frequency, i.e., a moderately-changing image signal, is compressed while an image signal of a high-frequency component having a frequency higher than the low-pass filter cutoff frequency, i.e., a busy image signal of large density variation in the image space, is maintained without being changed, thus enabling compression of the dynamic range of the original image as effectively as the conventional dodging and enabling an image to be suitably reproduced throughout the entire image area so that the image is closer to a viewer""s impression of the original image even if the original image is a backlighted scene, a high-contrast scene, or the like.
The above-described processing methods, however, entail a drawback in that, since a low-frequency component of an image signal is compressed by using a low-pass filter, if the ratio of dynamic range compression is high, a pseudo edge band of a certain width is generated along an edge of a subject at a boundary between the subject and the background at which the image signal changes abruptly, resulting in failure to faithfully reproduce the original image. To avoid generation of such a pseudo edge band, the degree of unsharpness of the unsharp image may be reduced (by increasing the cutoff frequency of the filter) to such a level that the pseudo edge band generated as described above is not visually recognizable. In this manner, the ratio of dynamic range compression can be reduced to limit an appearance of the above-described pseudo edge band. However, this method of reducing the ratio of dynamic range compression by reducing the degree of unsharpness of the unsharp image has an adverse effect. That is, when the processed image signal generated on the basis of the unsharp image is added to the image signal of the original image, a comparatively flat region, which has a relatively low contrast in comparison with a boundary region between the subject and the background in the original image at which the image signal changes abruptly by a large amount (having a large dynamic range) and has a relatively small change in density in the image space, may be further reduced in contrast to form an indistinct and unsharp flat region.
In view of the above-described circumstances, an object of the present invention is to provide an image processing method and apparatus which process an image by dynamic range compression processing to achieve the same effect as the conventional dodging without losing the desired sharpness of the image even if the processed image is a high-contrast wide-dynamic range image, such as a backlighted image or an image photographed by using flashlight, which thereby output an image signal to reproduce a high-quality image, and which output an image signal capable of reducing the appearance of a pseudo edge band caused when the ratio of dynamic range compression is high.
To achieve the above-described object, according to one aspect of the present invention, an image processing method having the following steps is provided. The step of generating a plurality of unsharp image signals representing unsharp images of an original image from an image signal representing the original image, the step of generating one synthesized unsharp image signal from the plurality of unsharp image signals, and the step of performing dynamic range compression processing on the image signal of the original image on a basis of the synthesized unsharp image signal such as to reduce the dynamic range of the original image.
It is preferable that the plurality of unsharp image signals are generated by filtering processes differing from each other in filtering characteristics and have different degrees of unsharpness.
Then, it is more preferable that the filtering processes are low pass filterings and differ from each other in cutoff frequencies.
And it is still more preferable that the plurality of unsharp image signals has a first signal and a second signal, the first signal being generated by a filtering process having lower cutoff frequency than that of a filtering process for the second signal, and the synthesized unsharp image signal is generated to have a value approaching to a value of the first signal as difference between the first signal and the second signal increases, and to have a value approaching to a value of the second signal as the difference decreases.
It is preferable that the filtering processes are performed by filter coefficients differing from each other and the filter coefficients are changed according to a number of pixels in an image reproduced as a reproduction of the original image.
And, it is also preferable that the filtering processes are performed by filter coefficients differing from each other and the filter coefficients are changed according to at least one of a result of scene identification of the original image and photography information relating to the original image.
In the image processing method, it is also preferable that the synthesized unsharp image signal is generated using a synthesis ratio or ratios of the plurality of unsharp image signals and the synthesis ratio or ratios is or are changed according to a number of pixels in an image reproduced as a reproduction of the original image.
And it is preferable that the synthesized unsharp image signal is generated using a synthesis ratio or ratios of the plurality of unsharp image signals and the synthesis ratio or ratios is or are changed according to at least one of a result of scene identification of the original image and photography information relating to the original image.
It is also preferable that the synthesized unsharp image signal is generated by synthesis from the plurality of unsharp image signals based on computation in accordance with at least one of addition, subtraction, multiplication and division of the plurality of unsharp image signals.
The present invention provides an image processing apparatus which performs dynamic range compression processing on an image signal representing an original image such as to reduce the dynamic range of the original image, the apparatus having a condition setting device for setting a condition of the dynamic range compression processing which has a number of unsharp image signals to be generated from the image signal of the original image, an unsharp image generating device generating a single unsharp image signal or a plurality of unsharp image signals from the image signal of the original image on a basis of the number set in the condition setting means, a synthesizer synthesizing the plurality of unsharp image signals into a synthesized unsharp image signal when the unsharp image generating device generates the plurality of unsharp image signals, and a processor performing the dynamic range compression processing on the image signal of the original image on a basis of the single unsharp image signal or the synthesized unsharp image signal.
In the image processing apparatus, it is preferable that the unsharp image generating device has a low pass filter or filters to be set which is or are variable in number and cutoff frequency or frequencies thereof is or are variable.
Then, it is more preferable that the low pass filters has a first filter for a first signal of the unsharp image signals and a second filter for a second signal of the unsharp image signals, the first filter having lower cutoff frequency than the second filter, and the synthesizer generates a synthesized unsharp image signal to have a value approaching to a value of the first signal as difference between the first signal and the second signal increases and to have a value approaching to a value of the second signal as the difference decreases.
It is also preferable that the cutoff frequency or frequencies is or are changed according to a number of pixels in an image reproduced as a reproduction of the original image.
It is also preferable that at least one of the number of low pass filters and the cutoff frequency or frequencies is or are changed according to at least one of a result of scene identification of the original image and photography information relating to the original image.
In the image processing apparatus, it is preferable that the synthesizer generates the synthesized unsharp image signal using synthesis ratio or ratios which is or are changed according to a number of pixels in an image reproduced as a reproduction of the original image.
It is also preferable that the synthesizer generates the synthesized unsharp image signal using synthesis ratio or ratios which is or are changed according to at least one of a result of scene identification of the original image and photographing information relating to the original image.